Circuit for controlling luminance signal amplitude

ABSTRACT

The circuit takes into account whether the image on a screen is too bright, whether more than one specific number of pixels have a luminance value that is greater than a given peak value and whether this condition is met in more than one specific number of lines in a picture and in more than one specific number of successive images with one such number of lines.

BACKGROUND OF THE INVENTION

The invention relates to the field of video systems, and in particularto a circuit for controlling luminance signal amplitudes for applicationin a video system.

Video and television systems generally have an automatic luminancecontrol (brightness control), which controls the average overallbrightness in dependence on the brightest positions of a picture. Theprimary purpose of automatic luminance control is to protect the picturetube by limiting the beam current.

In picture-in-picture (PIP) systems, two pictures are displayedsimultaneously on one television screen by overlaying a smaller,secondary picture on a main picture. A problem with PIP systems is thata bright secondary picture often causes the automatic luminance controlsystem to reduce the brightness of the main picture. As a result, thepicture is properly adjusted for the brightness of the secondarypicture, but this adjustment is often too dark for the main picture.

Therefore, there is a need for a circuit for controlling luminancesignal amplitudes to control the brightness of the image in the mainpicture effectively independently of the brightness of the image in thesecondary picture.

SUMMARY OF THE INVENTION

It is an object of the present invention to create a circuit to controlthe luminance signal amplitude in video systems, and thus picturebrightness. Especially in the case of picture-in-picture systems, theinventive circuit controls the brightness of the main picture,independent of and uninfluenced by the brightness of a secondary ormini-picture which is cut into the main picture.

The first device may include a first counter to count the pixels in aline which exceed a prescribed luminance threshold, and a second counterto count the lines in a picture, in which the status of the firstcounter exceeds a certain pixel-number threshold. The first device mayalso include a third counter to count the pictures in which the statusof the second counter exceeds a line-number threshold.

The invention is especially based on the insight that the picture shownon a screen is shown too bright (i.e., a viewer perceives it as toobright), if more than a certain number of pixels have a luminance whichis greater than a specific peak value, and if this condition prevails inmore than a certain number of lines in a picture and in more than acertain number of successive pictures.

The circuit is preferably implemented as a digital circuit, and as partof the (digital) picture-in-picture subassembly.

The first device may include a compare the luminance of a pixel with aluminance threshold, the first comparator then actuating the firstcounter. The first device may also include a second comparator tocompare the counter status of the first counter with the pixel numberthreshold, the second comparator then actuating the second counter. Inaddition, the first device may include a third comparator to compare thecounter status of the second counter with the line number threshold, thethird comparator then actuating the third counter, in such a fashionthat its status is incremented by one if the status of the secondcounter exceeds the line number threshold, and is decremented by one ifthe status of the second counter falls below this threshold.

The second device preferably includes a fourth comparator and a unit tocreate incrementally increased or reduced luminances, if the counterstatus of the third counter exceeds a first limit value FLDO or fallsbelow a second limit value FLDU. In particular, the first limit valueFLDO can have a value +2, and the second limit value FLDU can benegative.

The luminance threshold and the first two limit values FLDO and FLDUpreferably are set through a bus, so as to afford optimal adaptation tothe TV system.

Further details, features, and advantages of the invention will beapparent from the following description of a preferred embodiment, interms of the drawing.

BRIEF DISCUSSION OF THE DRAWING

The FIGURE is a block diagram illustration of a preferred embodiment ofthe invention.

DETAILED DISCRIPTION OF THE INVENTION

The FIGURE illustrated a block diagram illustration of a circuit forcontrolling luminance signal values. The circuit includes an adder 10that includes an input that receives the luminance signals of the imagepixels Y_(IN), an inverting second input that receives an incrementalluminance Y_(INKR).

The output of the adder is connected to a first comparator 11, whichcompares the luminance of a pixel with a luminance threshold (e.g.,seven bit peak value), which can be set through a bus (I²C bus, notshown). Furthermore, a clock pulse signal CLKLMC is applied to the firstcomparator 11, which is matched to the input signal Y_(IN).

The output of the first comparator 11 is connected to a first counter12. The comparator increments the counter status by one if the luminanceof a pixel exceeds the luminance threshold.

The status of the first counter 12 is evaluated by a second comparator13, which increments the status of a subsequent second counter 14 byone, if the status of the first counter 12 exceeds a threshold of thepixel number, that is if more than sixty-four (64) pixels are present inone line, whose luminance exceeds the luminance threshold.

The first counter 12 and the second comparator 13 are reset orconfigured by a signal LINE1 at the end of each active line.

The value of the second counter 14 indicates the number of lines in apicture, in which the luminance of more than sixty-four pixels exceedsthe luminance thresholds. This number of lines is evaluated by a thirdcomparator 15, which increments a subsequent third (picture) counter 16by one, if the number of lines exceeds the value sixteen (16) (linenumber threshold), and it decrements this counter by one, if the numberof lines is less than the line number threshold.

A fourth comparator 17 determines whether this number is greater than afirst limit value FLDO (first criterion) or smaller than a second limitvalue FLDU (second criterion). The first limit value FLDO is heregenerally different from the second limit value FLDU. The limit valueFLDO can be set through a bus (I²C bus).

If the status of the third counter 16 fulfills the first criterion ofthe fourth comparator 17, a unit 18 increases the incremental luminance.On the other hand, if the counter 16 status fulfills the secondcriterion, the incremental luminance is reduced. The output signal ofthe unit 18 is fed back via a limiter 19 and a register 20 to the secondinput of the adder 10. The limiter 19 prevents the luminance from beingless than zero or greater than a fixed prescribed value. The step widthY_(SCHRITT) of the increment or decrement of the luminance is a positiveinteger.

The second counter 14, the third comparator 15, the fourth comparator17, and the register 20 are reset and configured by a signal VFLD at theend of each picture (or half-picture in interlaced operation).

The first counter 12 (pixel counter) and the second counter 14 (linecounter) operate as absolute value counters, while the third counter(picture or half-picture counter) operates from −N to N−1, where N is apositive natural number.

The uncorrected luminance signal Y_(IN) applied to the adder 10, and thecorrected luminance signal applied to the output of the adder, are bothconducted to a switch 21 that can be actuated by a switching signal S.The switch allows the circuit of the present invention to be selectivelyenabled/disabled. Depending on the switch position chosen by the switchsignal, either the corrected or uncorrected luminance signal Y_(OUT) ispresent at the output of the switch 21. This is conducted to further,generally known, signal processing steps.

In supplementary fashion, it should be noted that the output signal ofthe register 20 optionally can be used to indicate the degree ofbrightness reduction. The signal takeoff point needed for this isdesignated by P in the figure. This signal may be used to adapt thecolor saturation to the reduced luminance.

What is claimed is:
 1. A circuit for controlling luminance-signalamplitudes, for application in a video system, comprising: a firstdevice to determine the brightness of a picture according to the numberof pixels exceeding or falling below a defined brightness threshold insuccessive pictures, and by a second device for incrementing ordecrementing the luminance-signal amplitude of the picture beingpresented, characterized in that the first device comprises a firstcounter to count the pixels in a line which exceed a prescribedluminance threshold, a second counter to count the lines in a picture,in which the status of the first counter exceeds a certain pixel-numberthreshold, and a third counter to count the pictures in which the statusof the second counter exceeds a line-number threshold, such that thesecond device increments or decrements the luminance-signal amplitude,if the status of the third counter exceeds or falls below thepicture-number threshold until, starting from the picture brightnessdetermined in the first device, the specified brightness threshold isreached at least approximately.
 2. The circuit of claim 1, wherein thefirst device includes a comparator to compare the luminance of a pixelwith a luminance threshold, the first comparator then actuating thefirst counter.
 3. The circuit of claim 2, wherein the luminancethreshold value can be set through a bus.
 4. The circuit of claim 3,wherein the first device includes a second comparator to compare thecounter status of the first counter with the pixel number threshold, thesecond comparator then actuating the second counter.
 5. The circuit ofclaim 4, wherein the pixel number threshold can be set through a bus. 6.The circuit of claim 5, wherein the first device includes a thirdcomparator to compare the counter status of the second counter with theline number threshold, the third comparator then actuating the thirdcounter, in such a fashion that its status is incremented by one if thestatus of the second counter exceeds the line number threshold, and isdecremented by one if the status of said second counter falls below thisthreshold.
 7. The circuit of claim 6, wherein the line number thresholdcan be set through a bus.
 8. The circuit of claim 7, wherein the seconddevice includes a fourth comparator and a unit to create incrementallyincreased or reduced luminances, if the counter status of the thirdcounter exceeds a first limit value FLDO or falls below a second limitvalue FLDU.
 9. The circuit of claim 8, wherein the first limit valueFLDO and/or the second limit value FLDU can be set through a bus. 10.The circuit of claim 9, comprising a limiter connected to the output ofthe unit to create an incrementally increased or decreased luminance tolimit the incremental luminance to a value between zero and a positiveinteger.
 11. The circuit of claim 10, characterized in that the seconddevice has an adder, into which an uncorrected luminance signal Y_(IN)and, as a correcting variable, the incremental luminance Y_(INKR) areinputted, and whose output is connected to the first comparator.
 12. Thecircuit of claim 10, wherein said circuit is configured and arranged tobe implemented as part of an integrated picture-in-picture circuit. 13.The circuit of claim 10, wherein the first counter comprises anabsolute-value counter.
 14. The circuit of claim 10 wherein the secondcounter comprises an absolute-value counter.
 15. The circuit of claim 10wherein the third counter comprises a counter configured to countbetween −N and N−1.
 16. A circuit for controlling luminance-signalamplitudes, for use in a video display system, said circuit comprising:means for determining the brightness of a plurality of pictures fordisplay on the video display system, comprising (i) first meansresponsive to each of the plurality of pictures, for counting the pixelsin a line which exceed a prescribed luminance threshold, and forproviding a first count signal indicative thereof; (ii) second meansresponsive to said first count signal, for counting the lines in apicture in which the value of said first signal exceeds a certainpixel-number threshold, and for providing a second count signalindicative thereof; (iii) third means responsive to said second countsignal, for counting the pictures in which the value of said secondcount signal exceeds a line-number threshold, and for providing a thirdcount signal indicative thereof; and means for adjustingluminance-signal amplitude if the value of said third count signal fallsoutside a picture-number threshold.
 17. A circuit for controllingluminance signal amplitude of a bit mapped display, comprising: meansfor determining the brightness of a plurality of bit mapped images fordisplay on the video display system, comprising (i) first meansresponsive to each of the plurality of bit mapped images that eachinclude digitized pixel luminance-signal amplitude data, for countingthe pixels in a line which exceed a prescribed luminance threshold, andfor providing a first count signal indicative thereof; (ii) second meansresponsive to said first count signal, for counting the lines in atleast one of said plurality of bit mapped images in which the value ofsaid first signal exceeds a certain pixel-number threshold, and forproviding a second count signal indicative thereof; (iii) third meansresponsive to said second count signal, for counting the number of saidplurality of bit mapped images in which the value of said second countsignal exceeds a line-number threshold, and for providing a third countsignal indicative thereof; and means responsive to digitized pixelluminance-signal amplitudes, for applying a brightness correction biasto said digitized luminance-signal amplitudes if the value of said thirdcount signal falls outside a picture-number threshold.